Most, not all, BMWs have a fuseable link just after the battery post connection, could be..... that has blown and is grounding the pos cable to the chassis. The pos cable should come from the batt under or thru the chassis to the pos post or jumper cable connection, that's under the hood. I'd unhook the pos cable from the battery and at the jumper cable post, thus eliminating that cable, and run a jumper cable from the post under the hood back to the battery. That way if the problem goes away you narrowed it down to the OEM cable or the fuseable link